(1) Field of the Invention
The present invention relates to a method of producing a sintered compact consisting mainly of high density boron nitride.
(2) Description of the Prior Art
Cutting and grinding materials having high hardness and toughness have been demanded with the progress of cutting and grinding technics. Diamond has hitherto been used as the cutting or grinding material, but diamond reacts with steel and is worn at the high-speed cutting and grinding. Therefore, the development of high density boron nitride having a high hardness, which hardly reacts with steel, becomes very important.
Boron nitride can be divided broadly into crystalline boron nitride and amorphous boron nitride. The crystalline boron nitride is further divided into high density boron nitride, which requires high pressure during its synthesis, and low density boron nitride, which does not require high pressure during its synthesis. The high density boron nitride includes wurtzite-structured boron nitride belonging to the hexagonal system and zincblende-structured boron nitride belonging to the cubic system. The low density boron nitride includes graphite-structured boron nitride belonging to the hexagonal system.
Wurtzite-structured boron nitride is a little lower in the hardness, but is higher in the compressive strength and toughness than zincblende-structured boron nitride.
When these high density boron nitrides are used as a cutting or grinding material, the boron nitrides are used in the form of a sintered compact of polycrystal. For example, in Japanese patent application publication No. 39,444/75 and No. 13,163/76 and Japanese Patent Laid Open Application No. 125,412/74 and No. 49,309/75, wurtzite-structured boron nitride is sintered at high temperature and under high pressure to obtain a sintered compact of polycrystal.
The stability of wurtzite-structured boron nitride under high temperature and high pressure is described in Japanese Journal of Applied Physics, Vol. 14, No. 10, pages 1605-1606 (1975). Wurtzite-structured boron nitride is stable within a range of lower than about 1,300.degree. C and not lower than 55 Kb. Therefore, wurtzite-structured boron nitride can be sintered within this range without losing the wurtzite structure. But, even when the sintered compact is apparently good, a small number of interstices still remain among the crystal particles, and the sintered compact is broken by a low load. Wurtzite-structured boron nitride is partly or wholly converted into zincblende-structured boron nitride under a temperature-pressure condition of not lower than 1,300.degree. C and not lower than 55 Kb. Further, the Japanese Journal of Applied Physics, Vol. 14, No. 10, pages 1605-1606 discloses that wurtzite-structured boron nitride is wholly converted into zincblende-structured boron nitride under a temperature-pressure condition of higher than 1,650.degree. C and higher than 60 Kb. Therefore, excellent sintered compacts of high density boron nitride having characteristic properties of wurtzite-structured boron nitride have not yet been obtained.
Further, boron nitride is oxidized to form boric acid anhydride B.sub.2 O.sub.3, although the amount is very small. Boric acid anhydride has a low melting point, and the presence of the anhydride in the sintered compact of boron nitride results in decreasing the performances of the sintered compact in use as a cutting tool or grinding tool, whose operating parts will have an elevated temperature. Therefore, the formation of boric acid anhydride is not desirable.
The inventors have found out that, when powdery wurtzite-structured boron nitride is sintered together with a boride of titanium, hafnium or zirconium or with a mixture thereof, the conversion of wurtzite structure to zincblende structure can be suppressed even under a temperature-pressure condition of not lower than 1,300.degree. C and not lower than 55 Kb, and a sintered compact of boron nitride having high hardness, compressive strength and oxidation resistance can be obtained.